1. Field of the Invention
The present invention relates to an optical transmitter, and more particularly to an optical transmitter using a Mach-Zehnder Modulator (MZM).
2. Description of the Related Art
FIG. 1 is a block diagram showing the basic construction of a typical Return-to-Zero Alternate-Mark-Inversion (RZ-AMI) optical transmitter using an MZM and a Delay Interferometer (DI), and FIG. 2 is a diagram showing processing signals of the RZ-AMI optical transmitter shown in FIG. 1. The RZ-AMI optical transmitter 100 includes a pre-coder 110, a modulator driver 120, a Continuous Wave (CW) laser 130, an MZM 140 and a DI 150.
The pre-coder 110 pre-codes and outputs binary data S11 which are input Non-Return-to-Zero (NRZ) signals. The modulator driver 120 receives the input from the pre-coder 110, amplifies it and outputs the amplified signal as a pre-coded signal S12. The pre-coder 110 may include a 1-bit delay element and an exclusive-OR element. The MZM 140 intensity & phase-modulates and outputs, according to the amplified signal, a light input from the CW laser 130. The bias position of the MZM 140 is located at a null point corresponding to a minimum value in a transfer characteristic function of the MZM 140. The DI 150 splits the modulated S13 input from the MZM 140 into a first and a second optical signal, delays the first optical signal by 0.5 bit, i.e., one half of a bit period, and outputs an optical signal S15 obtained by combining the first delayed optical signal and the second optical signal so that they destructively interfere. Then, an RZ-AMI optical signal is obtained by phase-modulating the destructively-interfered optical signal S15 each bit by means of a phase modulator so that the optical signal S15 has an inversed phase. The RZ-AMI modulation scheme known in the art has characteristics in which an optical signal includes intensity information and a phase of the optical signal is inverted alternately with each bit. In particular, in indicating the intensity of an RZ-AMI optical signal, as in the case of an RZ signal, a shift in energy of the RZ-AMI optical signal from a level 0 to a level 1, with a subsequent return to the level 0, indicates a single bit. Accordingly, since the RZ-AMI optical signal has the same change of intensity as that in the RZ signal, the RZ-AMI optical signal has an advantage in an RZ modulation scheme. For example, the RZ-AMI optical signal is tolerant to a non-linearity of an optical fiber in a transmission system having a data speed more than 20 Gb/s. Further, since the phase of the optical signal is inverted alternately each bit, a frequency component of a carrier is suppressed. Therefore, the RZ-AMI optical signal is tolerant to not only the Brillouin non-linearity effect but also the non-linearity effect such as the Intra-channel Four-Wave-Mixing (IFWM) and the Intra-channel Cross-Phase-Modulation (IXPM).
However, the RZ-AMI optical transmitter 100 as described above is expensive due to the expensive parts, particularly the MZM 140, the DI 150 and the phase modulator. Therefore, a system with the RZ-AMI optical transmitter 100 tends to require a non-competitive price.
Since a chirped RZ signal has been known to be tolerant to the non-linearity effect, it is observed by the present inventors that the RZ-AMI optical signal may also have the same advantages. Accordingly, a chirped RZ-AMI modulation scheme may be a very superior modulation scheme having advantages of a chirped RZ signal and an RZ-AMI optical signal. What is needed is a chirped RZ-AMI optical transmitter that is low-priced and tolerant to the non-linearity effect.